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The US Centers for Disease Control and Prevention now recommends people wear cloth masks in public places.

With cases of COVID-19 growing rapidly in the US and mounting evidence that the virus responsible, SARS-CoV-2, can be spread by infected people before they develop symptoms, the US Centers for Disease Control and Prevention recommended on April 3 that people wear cloth face coverings in public places. This guidance is a shift from the center’s previous position that healthy people only needed to wear masks when caring for someone who is sick. The recommendation also follows recent calls by experts on social media and other platforms for the general public to don nonmedical, cloth masks to help reduce the transmission of the novel coronavirus.

“Members of the general public should wear nonmedical fabric face masks when going out in public in one additional societal effort to slow the spread of the virus down,” Tom Inglesby, director of the Johns Hopkins Center for Health Security, tweeted on March 29.

These experts hope the measure will reduce the rate of disease transmission by adding an additional layer of protection in places where social distancing is difficult, such as grocery stores, while reserving limited supplies of medical-grade protective equipment for health-care workers.

The internet is exploding with mask-sewing patterns and advice on which materials are best to use, but many unanswered questions remain about how exactly SARS-CoV-2 spreads and what benefit widespread wearing of nonmedical masks may offer individuals and the public. Because of the inherent variability in household materials, mask design, and mask-wearing behavior, experts caution that the practice is no replacement for social distancing.

“It is critical to emphasize that maintaining 6-feet social distancing remains important to slowing the spread of the virus,” according to the CDC’s web page on the use of cloth face coverings.

Understanding what a mask needs to do to protect the wearer and those around them begins with understanding how SARS-CoV-2 spreads. Experts think people pass the virus to others primarily through respiratory droplets. These infectious globs of saliva and mucus, expelled by talking and coughing, are relatively large and travel limited distances—they tend to settle on the ground and other surfaces within 1–2 m, although at least one study has suggested sneezing and coughing can propel them farther (Indoor Air 2007, DOI: 10.1111/j.1600-0668.2007.00469.x). Scientists have not yet reached a consensus on whether SARS-CoV-2 virus can also spread through smaller aerosols, which have the potential to spread farther and linger in the air. In one experiment, researchers found that the virus can remain infectious in aerosols for 3 h in controlled lab conditions (N. Engl. J. Med. 2020, DOI: 10.1056/NEJMc2004973). But this study has limitations. As the World Health Organization noted, the researchers used specialized equipment to generate the aerosols, which “does not reflect normal human cough conditions.”

Homemade and other nonmedical cloth masks would function like surgical masks, which are designed to minimize the spread of the wearer’s germs to surrounding people and surfaces by blocking respiratory emissions from the wearer. Respiratory emissions include saliva and mucus droplets, as well as aerosols. These masks, often made of paper or other nonwoven materials, fit loosely around the face and allow air to leak in around the edges when the user inhales. As a result, they’re not considered reliable protection against inhalation of the virus.

In contrast, tightly fitting N95 masks are designed to protect the wearer by trapping infectious particles in complex layers of extremely fine polypropylene fibers. These fibers are also electrostatically charged to provide extra “stickiness” while retaining breathability. N95 masks, which if used correctly can filter at least 95% of small airborne particles, are critical for the safety of health-care workers who are regularly encountering infected people.

Credit: Stanford University/4C Air

N95 masks are made of layers of extremely thin polypropylene fibers, which can be seen in this scanning electron microscope image.

The ability to block respiratory emissions—as cloth masks and surgical masks can—is important because of growing evidence that people who are infected with SARS-CoV-2 but who have mild symptoms or are asymptomatic can unwittingly spread the virus.

“One of the challenges with the virus that causes COVID-19 is that sometimes people can have very mild symptoms that they may not even notice, but they’re actually highly infectious,” says Laura Zimmermann, the director of clinical preventive medicine for the Rush University Medical Group in Chicago. “And so they’re actively shedding the virus and can potentially infect others.”

Zimmermann says members of the Chicago health-care community have discussed the potential to distribute fabric masks to sick patients rather than surgical masks, to conserve personal protective equipment (PPE) supplies. “The cloth mask can really help out if somebody has some kind of infection, and you’re trying to basically contain the droplets,” she says.

In a recent communication, an international team of researchers reports that surgical masks can significantly reduce the amount of virus released into the air by people with respiratory illnesses, including infections by other coronaviruses (Nat. Med. 2020, DOI: 10.1038/s41591-020-0843-2).

Some experts encouraging widespread wearing of nonmedical masks point out that some countries that have successfully controlled their outbreaks also deployed this practice. “Face masks are used widely by members of the public in some countries that have successfully managed their outbreaks, including South Korea and Hong Kong,” according to a March 29 report on the US coronavirus response from the American Enterprise Institute.

Linsey Marr, an expert in airborne disease transmission at Virginia Polytechnic Institute and State University, says her thinking has evolved in recent weeks, and she no longer thinks only sick people should wear masks. Although some face masks may help reduce the wearer’s exposure to viruses, she says, the primary goal would be to reduce the spread of SARS-CoV-2 from infected individuals.

“If everyone wears masks, then less virus will be spread through the air and on surfaces, and the risk of transmission should be lower,” she wrote in an email to C&EN prior to the CDC’s new recommendation.

But people considering making their own mask are faced with many options in design and fabric choice, and it may not be easy to determine which options would be most effective. Neal Langerman, a chemical safety expert who is currently advising companies on coronavirus protective measures, notes that the permeability of household materials can vary widely and in unpredictable ways, making it hard to determine definitively which material is best for a homemade face mask. How tightly a material is woven can be a factor, as well as the type of fibers used. For example, natural fibers can swell when exposed to moisture from a person’s breath, changing the fabric’s performance in unpredictable ways. There’s also an inherent trade-off between the size of pores in the fabric and breathability—the least porous materials will also be harder to breathe through. The manufacturer of Gore-Tex, a lightweight, microporous material commonly used for outdoor clothing, received a flurry of inquiries about whether the material would effectively filter SARS-CoV-2. The company released a statement warning against using the material for homemade face masks because of insufficient airflow.

“The difficulty is that different fabrics have different specifications, and there seem to be so many options on the market,” Yang Wang, an aerosols researcher at the Missouri University of Science and Technology, tweeted. Wang is among the researchers collecting preliminary data on the filtration of nonmedical materials in light of the current outbreak.

Scientists have previously raised the idea of using improvised masks to counter a quickly spreading viral disease, and severalexistingstudies have evaluated the filtration efficiencies of various household materials. One study of commonly available fabrics, including multiple types of T-shirts, sweatshirts, towels, and even a pocket square, found the materials blocked between 10% and 60% of aerosol particles similar in size to respiratory emissions, which is in line with the filtration efficiency of some surgical masks and dust masks (Ann. Occup. Hyg. 2010, DOI: 10.1093/annhyg/meq044). Which improvised material filtered particles the best varied depending on the size and velocity of test particles. Studies also note that a mask’s fit and how it’s worn can drastically impact its effectiveness, something that is difficult to replicate in lab conditions.

The CDC recommends using multiple layers of fabric to make a face covering. In a video, US Surgeon General Jerome Adams demonstrates how to make such a mask from items found around the home, such as an old T-shirt.

In spite of the variability in homemade mask effectiveness, there is some evidence that even a partial reduction in particle spread can help reduce the rate of disease transmission across a population. In a 2008 study, researchers in the Netherlands found that although improvised masks were not as effective as personal respirators, “any type of general mask use is likely to decrease viral exposure and infection risk on a population level, in spite of imperfect fit and imperfect adherence” (PLOS One 2008, DOI: 10.1371/journal.pone.0002618).

Langerman says his primary concern related to the general public wearing masks is that, as with any PPE, using a face mask can give the wearer a false sense of security, and they may be less rigorous with other precautions. Experts have reiterated the importance of maintaining a physical distance of 6 ft (1.83 m) or farther from other people, whether they are exhibiting symptoms or not. Langerman cautions against placing too much trust in homemade fabric masks to protect oneself or others.

“That’s what this comes down to,” he says. “If a person’s going to make their own respirator, do they fully understand the risks in their selection, so that at least they know what the compromises are that they have opted for? I’m not sure that the answer to that will be yes.”

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Comments

David Lini (April 8, 2020 11:49 AM)

You really did not answer the question. You said a lot of people are talking and studying the issue, typical scientist. But you did not say that A is better than B or C. A chemical engineer would have provided some alternatives which are reasonable for the situation we are in since the 3M masks aren't available to thepublic.

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Hemant Desai (April 8, 2020 2:54 PM)

I agree David.Has anyone assessed cotton, polyester or nylon materials?They could be assessed by seeing if rubbing with polyacetate, polyethylene rods or materials can impart a charge which may deflect an aerosol drop if similarly charged or absorb if oppositely charged.I would expect nylon handkerchiefs might be suitable.Also, I have suggested these materials are salted ie soaked in saturated table salt solution, dried, charged by gentle rubbing then worn outside but when people return/come inside they are washed in hot water and detergent. They can then be re-treated for next time. If used indoors, I am guestimating they may be effective for 2 hours, I have no data to support this but fresher the mask the more effective.Tests would help.

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nancey sciancalepore (April 28, 2020 4:53 PM)

When you say, "Also, I have suggested these materials are salted ie soaked in saturated table salt solution, dried, charged by gentle rubbing then worn outside but when people return/come inside they are washed in hot water and detergent. They can then be re-treated for next time." Is there a certain fabric that must be used for this to work? I was thinking of trying denim/cotton. Thanks

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Bob Buntrock (April 29, 2020 11:05 AM)

Quilters know that the best fabric is cotton fabric for quilting. My deceased wife, a prolific quilter, left quite a stash when she passed away. My niece cleaned out many of the pieces a couple of years ago for her church's quilting projects for overseas quilts but left many smaller pieces. My daughter is no a quilter but an excellent at sewing and she recently got many of the smaller pieces to make face masks for herself and her husband who run a counseling service and still must see several clients onsite (at 6 feet distance of course). The masks are washable and she has a good pattern.

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Herb Seltzman (April 9, 2020 10:48 AM)

Has there been consideration of using a Quaternary ammonium disinfectant-infused covering on a DIY face mask? It is not the weave size but the disinfectant property of such a filter (e.g. Chlorox Wipes) that might be an effective barrier. The volatility of such quad salts are low and are considered non-hazardous to lung tissue should any even reach the lungs.

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4TimesAYear (May 13, 2020 8:54 AM)

Pretty sure that would be detrimental to one's health. I have a cannister of Clorox wipes right here. Two things worth noting: "It is a violation of Federal law to use this product in a manner inconsistent with its labeling.""Avoid contact with eyes or clothing. Wash thoroughly with soap and water after handling."

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Benton Bowman (April 9, 2020 6:24 PM)

Vacuum cleaner bags. Dozens of manufactures make 'Type Y' bags. They trap 99.7% of dust, mold, pollen. Better than a N95. Any place that sells a vacuum cleaner sells vacuum cleaner bags. They have regular less expensive bags. Look closer you'll find very nice premium bags. Yes I understand no mask is 100%. You can fit 100 million Coronavirus's on the head of a pin. But your not trying to stop a microscopic particle. Your trying to stop a sneezed out droplet from being inhaled. Probably more important is containing your sneeze. A mask is also a good reminder, don't touch face, wash hands often. Stay well, Ben B

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nyc (April 11, 2020 7:06 PM)

But have to be careful about which brands won't release micro fibers--this was addressed in an article I came across recently--sorry forgot where I read it, but something people should know to carefully select the proper bags where inhaling through them is not dangerous.

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Ben (April 13, 2020 12:16 PM)

Yes. Carefully check the type of vacuum cleaner or shop vac filter used. Many types use fiberglass or other potentially harmful materials. Ease in breathing through the fine filter material is also an issue.

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Jethro (April 26, 2020 7:27 PM)

I dont understand why its still being said that the only way to spread the virus is through droplets, even though at this point it should be also understood that the virus can be airborne and can stay up there for a long time. Trying to stop the droplets is problematic as it does not face the problem of the virus being airborne. Additionally, here is also the issue of homemade masks that if a droplet will hit your mask then it will stick if its a cloth or even if its a vacuum bag, thus increasing the chance of infection as the droplet will get absorbed by the material.

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Nobody (May 5, 2020 6:27 PM)

That's the point of the charged filter media. Any fabric can and will become electrostatically charged from simple friction - the friction of layers of fabrics touching each other is plenty to create enough charge to allow viral particles, which have an inherent negative charge, to cling to the positively charged fibers, thus becoming "trapped" within the material. Once attached, they don't release except by mechanical or chemical action, ie, cleaning with water, immersion in detergents - which dissipate the charges. This is generally accompanied by the destruction of the lipid outer layer of the particle, so it is then no longer viable anyway.

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KathyA (May 14, 2020 2:52 PM)

I purchased some unwoven polypropylene material that I have lined my cotton fabric masks with. I am hoping this lining will do the trick in helping prevent the virus getting through. As much as everyone says we wear masks to protect everyone else, let’s be serious. We all want to protect ourselves as well. Wish this article touched on which specific fabrics were best at blocking coronavirus particles.

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C S (May 26, 2020 7:03 PM)

That’s what I am trying too, but unsure of how many layers to use and mine is disposable. How have your masks worked out for you. Do you take a filter in and out as well? Had to have something to keep the mask cool in this hot weather.

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Christopher (April 9, 2020 6:25 PM)

Great information! I'm one of the people who thinks everyone should be ideally wearing a mask and any mask is better than none, but ideally they have the most protective mask, but DIY is becoming a thing as shortages are everywhere right now.

There is still too much mask misinformation - I still see a lot of people debating whether masks are effective or not... oh boy!

People can be asymptomatic meaning that they don't know they have the virus as they have no symptoms, meaning they can pass it around freely without knowing.

If everyone was wearing masks then the infected people would be protecting the world by not exposing other people to the virus, especially elderly people.

If anyone is interested I wrote an article on a similar subject comparing R95 vs N95 Face Masks: Which is More Protective?https://wellnessnova.com/r95-vs-n95-face-masks/

Stay safe everyone, and stay informed - new information is coming in daily!

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Hemant Desai (April 10, 2020 1:12 AM)

Improvised face mask + saturated Salt solution Soak.You can do this with handkerchief but first soak it in saturated salt solution, then dry. You can soak, scarves, snoods, existing face masks. Not best fit but better than nothing. Advantage of clothes, handkerchief is that you can wash in hottest temperature water and detergent then Re-Use. If you have N95 masks or something that fits you, then you that in preference. To make saturated salt solution just get a pan or container with as much water as you need, I would start with 200ml then keep adding table salt (~60g) stirring until no more dissolves. That is now saturated. Charge to outer surface of improvised mask could be added by rubbing. We in the UK have to culturally change and wear face masks. I wrote that on 21 March. Just seen Sanjay Gupta showing the same but not mentioning salting, disinfecting or charging.

We need to pass message to developing countries especially those in slums and cramped refugee conditions etc.

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Paul Rands (April 10, 2020 8:33 PM)

Good article but best answers about materials for improvised face masks came out in the comments, not in the piece itself. Is anyone aware of comparative studies/estimates/guesses which common household materials should work best? For example, I have cooling sweat band that can cover my mouth and nose. It's made out of some sort of microfiber. I'm sure it's better than noting but how likely is it that it's better than two layers of a cotton t-shirt?

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Elissa murphy (April 14, 2020 2:12 PM)

My daughter did an experiment during high school, took her to International Science Fair. It concerned hospital acquired infections and which fabrics were most likely to transmit bacteria. Don't know if this will help, but it was found polyester/plastics were most likely to transfer bacteria, because it would sits on top of the fibers. This leads me to lean towards using microfiber fabrics to stop spit droplets from seeping into the fabric. However, there are so many other things to consider. Damp masks are not effective, they should be changed often. Handling of the mask, taking off, applying, and type of filter, if any, all affect performance. Also, what is the fit of mask...air takes the path of least resistance. Why would air go through a filter when it can simply go around the mask. Personally, I'm opting for a microfiber outer fabric, removable stabilizing fabric filter and flannel inner layer with nose clip. I may add adhesive used for holding up compression stockings if need be to seal down the mask as much as possible. I work as an "essential" (previously expendable) worker at a large big box store. People apparently do not understand the 6' rule, what "stay at home" means or what an "essential" product really is.Good luck.

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Lesley (May 13, 2020 9:44 AM)

A wise contribution Elissa. Have you any ideas on what to use for the nose clip part? Wearing glasses means a good fit over the nose is important to stop foggin up too. What would you call 'stabilizing fabric filter'?

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Ssizzling7s (May 26, 2020 5:42 PM)

I’ve tried many different types of flexible metals for the nose piece, and the best is likely one you already have...coat hangars. They hold their shape and can be manipulated with pliers. Just see a sleeve into your mask for these to fit into. I wrap them in duct tape so they don’t poke through the fabric.

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Erika (May 13, 2020 11:52 AM)

I know I'm late to this conversation, and I have no affiliation with this website, but I found some great info digging around here: https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/ They tested various materials for filtration, air flow, and and what happens when they get moist. The data are presented on different pages, so you have to kind of click around, but it's the most scientific review of household and DIY mask materials I've found. So I made masks for my family from two layers of cotton T-shirt fabric, as it's all I had, and now I have some high-threadcount woven cotton to make better ones.

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Suzy (May 26, 2020 9:51 AM)

Thanks for this and everyone! It is so hard to find out what chemicals some of the HEPA filters have warnings for, and about. Personally I think it’s the melt/heat chemicals but those are likely inert or not in any quantity to hurt. It’s a trade-off.But there are also pollen and dander vacuum bags that claim 99.7% and don’t have the California warning. And contain carbon cloth. . I’m going with those in a washable cotton sleeve.

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William Hamm (April 10, 2020 10:26 PM)

Another alternative to home made masks is re use. MIT , Harvard and Mass General studied the use of gamma radiation on N95 masks. Too bad the filtration efficiency went down too far, but other equipment could be sterilized this way. Via communication with a relative who is a nurse at Boston Children's Hospital, they doing this now. I have no information other than hearsay. MIT and others suggested just ageing masks for a few days in a paper, saving masks, saving lives. Stanford investigated the use of ovens and found that 30 minutes at 158 degrees F worked and didn't degrade the filtration efficiency of the mask. Battelle Memorial Institute in Ohio developed a chemical process good for a wide variety of PPE that involves a big chamber and the use of hydrogen peroxide gas. They are deploying the system to hot spots. The Mayor of Somerville Mass recently announced the installation of this process in an empty retail center. There is a lot of current information on reuse, which has a lot of other dimensions than sterilization such as fit, etc. and I find it difficult to imagine offering an ICU worker anything but new equipment, but establishing a pipeline from that ICU worker to bus drivers, etc. seems like it's worth a lot of effort

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I have to look into these "salt" ideas some of you mention so a charge can be applied. Just concerned how that could affect your skin and lungs if worn for any real length of time.

I'm a nurse--NOT a chemical engineer! Any of you understand why silk is considered "bad" for masks? It's very easy to rub and make a charge with!!

I'm also considering wool felt. Wool is antimicrobial, and by felting, fibers become rough, stick together in ways similar to hook & loop (Velcro). I'd think a layer of felt might have decent filtering ability, and I think can also be charged?

It's easy to create a good seal with a snug fitting mask. But tricky to do that while also keeping the mask pretty dry. A lot of "home made" mask ideas involve a snug fit that stretches over nose & lips and are likely to build up exhaled moisture. My understanding is that wet masks no longer filter/prevent viral pass thru.

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Iac (April 13, 2020 12:34 PM)

Re Silk, I think it doesn't filter really at all, because of how thin and loose the weave is.On a test chart I saw, it was at the Bottom re, effectivness (sp?).Don't know if salting would work much.Re Felt Wool, it would be more difficult to breath through than an N95. Especially if walking etc. On the same chart showing Silk at the bottom, two layers of cotton-synthetic cloth worked rather well !

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Hazel (April 14, 2020 9:47 AM)

This is an interesting informative article https://www.google.co.uk/amp/s/www.nytimes.com/article/coronavirus-homemade-mask-material-DIY-face-mask-ppe.amp.html that will inform you about the effectiveness of filters and fabrics for homemade masks

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KariNurse (April 12, 2020 2:30 PM)

Help--Chemical Engineers! Is there a squeeze, easy application silicone product that is safe, non toxic against skin, that can be applied to a surface, cure, and be soft, flexible, slightly sticky afterward (like the silicone that helps hold up some socks, women's no show/backless bras, husbands to prevent them from sticking)?

I have a hard time getting most N95s to "fit". Size small duckbill styles fit me best, I've often had to use Hepa Hoods with PAPRs in the past. Going to hotspot ICU next week. I have some N95s that "almost" fit. Wondering if there's a silicone I could squeeze around edges of mask and mold to my face to get seal. Plan to rotate & reuse a weeks worth.

What is that slightly sticky, super flexible silicone anyway? Does anyone know what I'm talking about? Maybe it could be used to make a better seal on home made masks that also use some sort of frame to keep the material from lying directly on your nose & mouth. 100% silicone caulk material won't work, already tried. Thanks!

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Amanda (April 13, 2020 10:11 AM)

You may want to check out a company called Smooth-On. They sell platinum cure silicones for the film industry, to make props and prosthetic / makeup appliances. Some may even come pre-loaded in a cartridge gun. Those kinds of silicones would be more skin friendly. I would NOT recommend using silicone caulks and such you may get at home improvement stores. There are also other silicone adhesives for things like toupees and other prosthetics. I think 'It Stays' is one of them. But the adhesion is pretty high, and removal could be painful. There could also be double sided tape for clothing, that may help.

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Elissa murphy (April 14, 2020 3:29 PM)

I'm sure "It stay's roll on body fixative" would work. I use it to hold up thigh high compression stockings. However, beware, it pulls taking off the fabric so if this is for short term, on off on off, not a good thing. If mask is being changed /disposed of , when taking it off, wet down the area where glue was applied...this really sticks. Find it online easy. Otherwise there is the hair piece tape, double sided, online. "Vapon", a medical grade tape seems to work for just about everything. I have read posts by doctors who are using it to get a better seal on masks. I would be sure you skin is oil free so it sticks well. My daughter's a nurse in a hospital, so I somewhat understand your concerns.

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none (May 5, 2020 6:38 PM)

A mask with a rubberized/polymer edge could be sealed with a thin layer of petroleum jelly (Vaseline), the disadvantage is this would tend to be a short term solution not for all day wear. Mechanical seals (ie, a tight fit) are probably just as good, and the best ones can even be made relatively airtight. The small amount of air that leaks from the skin-mask interface could be alleviated with a positive pressure apparatus, all it would require is a backpack canister feeding a cannual that is providing positive pressure from the interior of the mask. Probably easier said than done -if you're going to go to the trouble and complicating of positive pressure masks, they already have those designs in the full bio-hazard suits for Level 3 and higher. The issue there is they really cut down on mobility and aren't designed for all day wear either.

Honestly I think most of these solutions are overkill for non-medical personnel. If you're simply worried about community exposure there is no need for anything more sophisticated than a well fitted surgical mask or N95 mask. The infectiousness is related to viral load, so cutting down on what you are going to be exposed to in the community by 80+ % is going to result in almost total reduction of actual risk. You'll still have some risk of droplet infection via the ocular route, but in terms of the respiratory route, a basic surgical mask is adequate to be preventative against "community exposure", of the sort you will get in public places or spaces ...the caveat being again that infectiousness is related to load, so the more time you are exposed to a given load, the higher the risk gets.

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Monetdog (May 7, 2020 4:17 PM)

Sounds like you are talking about Silly Putty (as available in little eggs toy stores) or 3179 Dilatant Compound (as available in 50lbs blocks from Dow). Physical therapists give it out too. Loktite Fun Tak, normally used to hold up posters, may also work.

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Skeptical (April 13, 2020 12:54 PM)

Masks don’t filter sub micron particles from getting into the air. Will everyone touting masks keep them on when they have a sneezing fit? How about when they are wet from perspiration and moisture from breathing, are the filtering properties the same? I doubt it. If asymptomatic people can infect, my guess is they ate not doing so by sneezing. I was hoping to get some scientific based facts out if this article, not opinions.

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Hemant Desai (April 14, 2020 12:44 AM)

KariNurse: I considered silicone to help make a rim to help create a flexible fit but I thought that might be difficult and not easily sourced in developing countries.Therefore, I considered clay in a strip stitched around the edges of a face mask or plasticine, play-dough or ideally something you can find easily, is non-toxic, can be washed, remoulded and re-used.Starching or in a hospital using broken bone setting material like plaster of paris might help to shape around your face. But I was not able to check.

Lupe: in my opinion, polyester and cotton should be viable. I have not read anything to say they are not good. The balance is getting enough air in and out, while trapping aerosol as effectively as possible.

Alternatively, if you have a CPAP mask which fits you well, then make your salted improvised face mask, put it on, and place CPAP on top to create a seal on top of mask. A thought, consider?

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Janet C (April 14, 2020 9:42 AM)

Researchers at Cambridge University did testing on a number of materials to aid in decision making regarding homemade masks: https://smartairfilters.com/en/blog/best-materials-make-diy-face-mask-virus/?rel=1Dish towels rated higher than other materials they tested, but they are working on testing moreHowever, some people say interfacing is a good liner. I don't think this has been tested. Most interfacings are non-woven, meaning that they are made from fibers bound together (think particle board), rather than woven.Another apparently untested option: Disposable shop towels made of pulp fiber and fresh tree fiber are pressed together, so smaller area for viruses to get through apparently. However they also may contain chemicals like latex resin, etc. so maybe not a good choice. a major factor is making the mask fit snugly. On the other hand it needs to be comfortable enough and breathable enough that you will wear it. The other day at the grocery store a guy came within two feet of me, talking away, with his mask around his neck. That won't help. :)

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patrice (April 14, 2020 10:33 AM)

The question about using spadnex.. not good..it stretches and when it does, there are wide spaces. The woman looking for face friendly caulking. I am using skin sensitive medical tape to hold the mask snug to my skin. it works great.

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Prof N J Parmar (April 15, 2020 8:45 AM)

It was nice to read the topic on face masks on ACS C and En. Information like this in the current corona pandemic situation all over the world really makes people like us to think what can be done best for the society. My curiosity is to have answers of few questions regarding this, if anybody can share I will be happy. I think, suppose cotton fabrics soaked with saturated solution of soap and dried, the network of threads thus adsorbed with suspended soap particles is kept as front layer of the face mask. Next immediately this is fatty oil-adsorbed cotton layer. And the third layer that is exposing to mouth and nose of simply dry cotton fabrics. The reason of keeping the middle layer oily is to allow air and not water from the upper next soap layer when inhaled in side. Keeping the front soap-adsorbed layer just wet works best to trap droplets and proteins of even virus. More is the adsorption of droplets on upper soap-layers, less will be the concentration of droplets in the air in the public place. And so less chance to spread the disease. If this works then often the failure of following the social distancing which results into more spreads of the COVID-19 can be controlled to some extent. This is my hypothesis and needed more advise of experts working in this area to improve working efficiency of house hold face masks.

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It probably will be but definitely only use one layer. It is best to test - make one, then wear it while you make more. Can you breath while sitting and not even moving around? I learned that with some masks I made with two layers of 911FF for a veterinary clinic. They cannot wear those specific ones for the day. With one layer they are fine. Your interfacing would be a little heavier, so hard to say 100%

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Canada Bob (April 16, 2020 12:30 PM)

I have made face masks from close weave cotton, but not wanting to have the outside of the mask touching the second layer, allowing droplets to soak through and be breathed in I bought a pack of 3M pan scourers, they are approx 8"x 6" and about 1/4" thick, so I cut them in half to 8" x 3" and inserted one piece as a sort of spacer between the layers of cotton, it works really well.

The spacer stops any dampness on either layer of the cotton wetting the other layer, that has to be good, add to that the pan scourers don't obstruct breathing and are easily washed in/with the mask.

The spacer makes it much less likely for a droplet in the air around me infecting me with Wu Flu.

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lisandra (April 26, 2020 1:43 PM)

Hello,I like the idea would you be able to make a video showing the steps of how you made it. Do I need a sewing machine. I am a mother of 2 and lost my job and I don't have any masks but I am trying to find ways to make the best ones I can for them especially myself because I am the one that goes outside when I have to.

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Canada Bob (May 2, 2020 2:43 PM)

Hello Lisandra,

I guess it would help to have a sewing machine, but there are simple fold methods that would work almost as well. I've recently incorporated another level of protection to my mask, a pretty simple one too, one that can be used on pretty well any mask or face covering.

I have sprayed my mask with a saturated solution of sodium chloride {regular table salt}. I use an empty spray bottle, fill it to 80% or so, then start adding the salt until the water won't dissolve any more salt, the salt just falls down to the bottom of the bottle, at that point you have made a saturated solution.

After giving the bottle a good shake to make sure you still have undissolved salt at the bottom of the bottle, you then spray your face mask or bandanna, the finer the mist from the spray the better. Let the material air dry, when the water has evaporated the salt will have formed nano crystals on and in the fibers of the material, cotton is probably the best material to use.

When you wear the "salt mask" IF a droplet falls onto your mask the salt in/on the mask will dry the droplet very quickly, this will dismantle / deactivate the Corona virus, or ANY other virus that falls on your mask.

This process is called Osmotic Shock, google it, the salt physically rips the virus apart.

Bob.PS: sorry to read of your plight, I hope that you and your family come through this safe and sound.

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Liz (May 3, 2020 9:28 AM)

Bob - THANKS for your post. Really appreciated. Can you share on the handling process of the salt mask? Specifically, should a salted mask be worn more than once? If so, how many times, without acceptable effectiveness loss?Thanks!Liz

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Canada Bob (May 6, 2020 1:49 PM)

Hello Liz,

Regarding how long a salt mask could be worn, well, I guess that's down to the materials used, and how the mask is constructed.

I use cotton with a 200 thread count, to make a double layer / pocket mask, basically it's just a fold over mask, but. Before I fold the material my wife sews in place a piece of Pellon 830 into the inside of the pocket, behind what becomes the front face of the mask, I hope you can follow my explanation ;-)

This really think sheet of material makes the front face of the mask "water resistant" for that read droplet and possibly even infected aerosol resistant.

When you fold the piece of cotton in half, to make the pocket that houses the piece of Pellon 830 from front to back you now have, cotton/pellon/cotton.

Pellon is easy the breathe through, but it is quite a decent barrier to water / infected droplets, it makes it difficult for a droplet to pass through the mask, it gets soaked up by the salt treated cotton on the front face of the mask. If you can't find Pellon 830 {a sew in place / non fusible material} you'll probably find a product called "Oly Fun" in craft shops. These materials aren't much thicker than a heavy grade of grease proof paper, but as mentioned above they are water resistant but they don't restrict the air that we need to breathe, that's kind of important in a mask.

There's two types of these materials, fusible {think iron on} and non fusible, sew on. just check what they have in the store gives that water resistance {far as I know all grades achieve that} and that you can breath through them, again I think they're all breathable.

Back to the how long can you wear the mask, that's down to the construction, if it's a sturdy mask you could wear it for months, but you would need to keep the salt crystals in place.

The masks that I have can easily be washed, at 60C or even hand washed, I leave them in the soapy water for a while {15 minutes}, then I rinse them in cold water to get the detergent out. I then let them air dry {overnight, or for a couple of hours}. At this point ALL the salt has been washed out, so them mask needs "recharging" with salt.

I do that using Lo-Salt, it's sodium and potassium, one salt helps the other, I use one of the empty spray bottles that we have around, just find one that gives a good misting spray. After you've washed the spray bottle out to get rid of any residue of what was in it, fill it to say 80% full with warm tap water, warm water absorbs more salt. Start pouring the Lo-Salt in {regular table salt is almost as good}, keep pouring until the salt starts to congregate at the bottom of the spray bottle, give it a good shake to make sure, if the salt is still at the bottom of the bottle you have made a SATURATED SOLUTION, the water can't absorb any more salt.

The result is, when you fine mist spray the dried mask the water will evaporate leaving the salt on and in the cotton fibres, they crystallise at microscopic level millions of little salt crystals all over the mask.

If a droplet comes your way and lands on your mask the droplet has nowhere to {easily} go, other than into the fibers of cotton on the front face of the mask, the Pellon restricts the droplet from moving from the from face of the mask to the back face. On the off chance that an incredibly small amount of an infected droplet get past the Pellon, guess what's waiting for in on the next layer, a minefield of salt coated cotton !!!

Back again to how long can you wear the mask, well, if you wash it say once a week, if you feel the need to, and the mask is sturdily constructed, I guess it would last months, 3 to 6 months maybe, but...

Every time you wash the mask you need to recharge it, by spraying it again, to keep your "minefield" in place. Not sure if you have read up on Osmotic Shock, how it physically RIPS the virus apart, it's very satisfying to know that's what's going on, it's certainly an interesting topic.

One last thing Liz, the more of the saturated solution you can get into the cotton fibers the better, I find that by adding a teaspoonful of Borax {a surfactant} to my salt solution the cotton seems to get "wetter" by getting wetter that means more solution is getting into the cotton fibers, the deeper the solution gets into the fibers the more salt crystals form, that's better protection for us, and major problem for the virus.

Chances are as long as your mask doesn't get wet, say in the rain, the salt would be good for a week, but I spray my mask every night, it's a pleasure to know that I'm laying more mines for the virus to land on.

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MissBee (May 17, 2020 5:06 PM)

Dear Bob.I had a cold a few years ago, and used a Himalayan salt inhaler and I developed bronchitis. How do you know how breathing such a high concentration of salt air is not going to affect your lungs adversely?I heard recently that many people are showing up at the hospital ER from inhaling household products which became toxic.

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Canada Bob (June 3, 2020 11:37 AM)

Hello MissBee...

I hear and I respect your concern, the thing is, the salts are infused into the fibres of the cotton, the only time they leave the mask is when you wash the mask, so unlike an inhaler you won't be ingesting any salt, they are effectively trapped in the mask.

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Hemant Desai (April 16, 2020 9:21 PM)

Fair point:See 1. www.nature.com/scientificreports OPENUniversal and reusable virusdeactivation system for respiratoryprotectionFu-Shi Quan1,*, Ilaria Rubino2,*, Su-Hwa Lee3, Brendan Koch2 & Hyo-Jick Choi2Aerosolized pathogens are a leading cause of respiratory infection and transmission. Currently used protective measures pose potential risk of primary/secondary infection and transmission. Here, we report the development of a universal, reusable virus deactivation system by functionalization ofthe main fibrous filtration unit of surgical mask with sodium chloride salt. The salt coating on thefiber surface dissolves upon exposure to virus aerosols and recrystallizes during drying, destroyingthe pathogens. When tested with tightly sealed sides, salt-coated filters showed remarkably higher filtration efficiency than conventional mask filtration layer, and 100% survival rate was observed in mice infected with virus penetrated through salt-coated filters. Viruses captured on salt-coated filters exhibited rapid infectivity loss compared to gradual decrease on bare filters. Salt-coated filters proved highly effective in deactivating influenza viruses regardless of subtypes and following storage in harsh environmental conditions. Our results can be applied in obtaining a broad-spectrum, airborne pathogen prevention device in preparation for epidemic and pandemic of respiratory diseases.Aerosols take a prominent role in airborne transmission of respiratory diseases. Droplets with aerodynamic size (da) < 10 μm and 10 < da < 100 μm are known to infect the alveolar regions and upper respiratory tract, respec- tively1,2. Notably, aerosols can also be a route of infection in diseases that, contrary to for instance influenza, do not specifically target the respiratory tract, as it could be the case of Ebola virus3. While vaccination can greatly reduce morbidity and mortality, during a pandemic or epidemic new vaccines matching the specific strain would be available, at the earliest, six months after the initial outbreak. Additionally, following development of an effec- tive viral vaccine, several potential problems would remain, such as limited supply due to insufficient production capacity and time-consuming manufacturing processes. As a result, individuals close to the point of an outbreak would be in imminent danger of exposure to infectious diseases during the non-vaccine period. In the absence of vaccination, respirators and masks can be worn to prevent transmission of airborne pathogenic aerosols and control diseases, such as influenza4.The main alternative, the N95 respirator, requires training prior to use, must be expertly fitted to address the risk of faceseal leakage at the face-mask interface, and must be disposed of as biohazard5. Due to these factors, the use of N95 respirators on a large scale is impractical and expensive during an epidemic or pandemic. Past experiences of severe acute respiratory syndrome (SARS), H1N1 swine flu in 2009, and Middle East respira- tory syndrome (MERS) indicate that surgical masks have been most widely adopted by the public as personal protective measure, despite controversy on their effectiveness6–9. Currently, among other factors, filtration in respirators and masks depends on filter characteristics, including fiber diameter, packing density, charge of fibers and filter thickness, as well as particle properties, such as diameter, density and velocity10–14. However, in the lack of a system to deactivate the collected pathogens, safety concerns naturally arise about secondary infection and contamination from virus-laden filter media during utilization and disposal. Furthermore, since re-sterilization is not possible without causing damage, respirators and masks are recommended for single use only9,15,16. Scientific efforts have been focused on treatment of filters with materials possessing well-known antimicrobial properties, such as iodine, chlorine and metals17–25, although with limited effectiveness against virus aerosols26–28. Therefore, a key challenge is the development of an easy-to-use, universal virus negation system, which is reusable without12 Department of Medical Zoology, Kyung Hee University School of Medicine, Seoul, 130-701, Korea. Department3 received: 04 August 2016 accepted: 30 November 2016 Published: 04 January 2017of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada.Department of These authors contributed equally to this work. Correspondence and requests for materials should be addressed to H.J.C. (email: hyojick@Biomedical Science, Graduate School, Kyung Hee University, Seoul, 130-701, Korea. ualberta.ca)* Scientific RepoRts | 7:39956 | DOI: 10.1038/srep39956 1

www.nature.com/scientificreports/Filtration efficiency against viral aerosols and protective efficacy in vivo. Filtration efficiency of salt-coated filters was tested against aerosols with volumetric mean diameter (VMD) of 2.5–4 μm containing H1N1 pandemic influenza virus (A/California/04/2009, abbreviated as CA/09) at different pressure conditions (see Fig. 2a for transmission electron microscope (TEM) image of H1N1 virus). Interestingly, as shown in Fig. 2b, Filterbare did not exhibit any significant level of resistance against penetration of virus under our experimental conditions (i.e., 0% filtration efficiency). Conversely, salt-coated filters showed substantially increasing filtra- tion efficiency with pressure and amount of coated salt. In particular, in the case of Filterwet+600μL, filtration effi- ciency varied from 43 to 70%, with increasing pressure from 3 to 17 kPa, and Filterwet+1200μL exhibited persistent, high-level efficiency (~85%) (one-way ANOVA, P = 0.85).To probe the effects of filtration efficiency on protective efficacy, in vivo experiments were performed using mice intranasally (IN) infected with penetrated dosages of H1N1 virus under breathing pressure (~10 kPa)29. As shown in Fig. 2c, similarly to negative control groups (mice infected with lethal dose of virus stock and aerosolized virus), mice exposed to a dose penetrated through the bare filter showed rapid body weight loss, followed by death within 10 days after infection, in good agreement with the observed 0% filtration efficiency (Fig. 2b). In contrast, mice groups exposed to virus derived from salt-coated filters resulted in 100% survival rate (Fig. 2d). Furthermore, lungs of mice from negative control groups exhibited severe lung infection 4 days after challenge (Fig. 2e). Conversely, mice groups exposed to virus derived from salt-coated filters showed significantly lower levels of lung viral titers (t-test, P < 0.005). This is consistent with lower levels of inflammatory cytokines, interferon-γ (IFN-γ), from salt-coated filter groups compared to negative control and bare filter groups (t-test, P < 0.001) (Fig. 2f ).Deactivation of virus on salt-functionalized filters. Influenza virus stability tests were performed to investigate the effects of salt coating. The same amount of recovered viruses from the PP fibers was used, and, in the case of bare filters, viral aerosols exposure was conducted in the absence of pressure due to 100% penetra- tion of viral aerosols. Unlike bare filters (Fig. S6a(i)), formation of micron-sized NaCl phase represents a typical feature of salt-coated filters due to recrystallization of NaCl salt, following local dissolution upon aerosols expo- sure (SEM images in Fig. S6a, ii to iv, and EDX mapping in Fig. S6b). In contrast to 8% HA activity loss of virus adsorbed onto Filterbare, salt-coated filters exhibited almost complete HA activity loss within 5 min of incubation (Fig. 3a). Such dramatic virus destabilization on salt-coated filters is further supported by negligible levels of viral titers compared to Filterbare with incubation time (t-test, P < 0.001) (Fig. 3b). It is also noted that virus titers exhibited significant decrease with increase of incubation time and amount of coated salt (ANOVA general lin- ear model, P < 0.001). TEM analysis showed that influenza virus on Filterbare experiences morphological change into non-spherical shape during aerosol drying (Fig. 3c(i)). Notably, influenza virus was severely damaged on salt-coated filters even at 5 min of incubation (Fig. 3c(ii)). From microscopic analysis, aerosol drying time was about 3 min, indicating that destruction of virus observed at 5 min is associated with drying-induced salt crystal- lization. Physical damage of virus due to crystallization was similarly reported as a major destabilizing factor of inactivated influenza virus30,31. Lower levels of native fluorescence and nile red fluorescence from virus recovered from salt-coated filters accounted for more severe conformational change of antigenic proteins and destabili- zation of viral envelope, respectively, consistent with TEM analysis (t-test, P < 0.001) (Fig. 3d). In parallel, we investigated the separate effect of salt concentration increase on virus stability during the aerosol drying process, irrespective of crystal growth. As displayed in Supplementary Fig. S7, the materials collected in suspension from Filterwet+600μL induced visible morphological transformation of the virus (Supplementary Fig. S7b) compared to suspension of Filterbare (Supplementary Fig. S7a). This can be attributed to the high salt/surfactant concentra- tion and osmotic pressure, which have been well-known to destabilize proteins and viruses31–33. Therefore, the marked virus destabilization on salt-coated PP fibers can be explained by the combined effects of salt concentra- tion increase during drying and evaporation-induced salt crystallization.To verify in vitro virus stability on the filters, an in vivo study was performed by infecting mice with virus incubated for 60 min on PP filters. As shown in Fig. 3e, Filterbare group exhibited 5% body weight loss at day 9 post-infection, reaching a body weight lower than that of salt-coated filter groups by 10–15%. Thus, significantly higher lung virus titers in the negative control group were observed in contrast to no detectable titers in the salt-coated filter groups (Fig. 3f).Strain-nonspecific virus deactivation and effects of storage under harsh environmental conditions on salt coating stability. Broad-spectrum protection of salt-coated filters against multiple subtypes of viral aerosols was evaluated by investigating both lethal infectivity by penetrated virus in vivo and infectivity by virus collected on filters during filtration in vitro using A/Puerto Rico/08/1934 (PR/34 H1N1) and A/Vietnam/ 1203/2004 (VN/04 H5N1). Similarly to CA/09 H1N1, 100% of mice survived viral infection (PR/34 and VN/04), with no evidence of weight loss, due to higher filtration efficiency of salt-coated filter than that of bare filter (Fig. 4a). This is supported by no significant level of viral titer in the lung. In addition, as shown in Fig. 4b, salt-coated PP filters destroyed adsorbed influenza viruses irrespective of both subtypes and amount of coated salts.The stability of salt coating on PP fibers was tested under harsh environmental conditions. Incubation at 37 °C and 70% relative humidity (RH) for 1 day did not cause any significant difference in filtration efficiency (t-test, P = 0.718) (Supplementary Fig. S8). As a result, all mice infected with dosage of penetrated virus through the filter stored at high temperature and RH displayed 100% survival with 7% of body weight loss (Fig. 4c,d). Even after 15 days of incubation, salts remained to coat PP fibers (Fig. 4e, and Supplementary Fig. S9a,b), despite change in grain orientation due to recrystallization (Fig. 4f, and Supplementary Fig. S10a,b). Scientific RepoRts | 7:39956 | DOI: 10.1038/srep39956 3

www.nature.com/scientificreports/DiscussionDevelopment of a universally applicable, low-cost, and efficient mechanism for virus negation is regarded as a major challenge in public health against general airborne biological threats. This led us to propose a new con- cept of personal/public preventive and control measures using salt-recrystallization against pathogenic aerosols based on two hypotheses. The salt-coating can enhance adsorption of virus on the filter fibers and inactivate virus by the increase of osmotic pressure followed by the crystallization of salts. As shown in Fig. 2b, salt-coated filters exhibited significantly higher levels of filtration efficiency than bare filters. Notably, the bacterial filtration efficiency (BFE) reported by the mask manufacturer is 99%. The different value of filtration efficiency for bare filters obtained under our experimental conditions may be partially due to the use of aerosols with different bio- logical origins. The FDA-recognized ASTM F2101 – 14 standard for evaluation of BFE exposes surgical masks to Staphylococcus aureus aerosols, by employing S. aureus ATCC 653834, which has an average diameter of about 1 μm. In this study, filtration efficiency was calculated following exposure of bare and salt-coated filters to influ- enza virus, which exhibits a smaller diameter than that of S. aureus by one order of magnitude. Additionally, whereas during BFE evaluation all three layers of surgical masks are used, in this work filtration efficiency refers to mask filters (middle layer). It is worth noting that the conditions for BFE standard evaluation (such as flow rate and time of application of flow) do not coincide with the experimental procedure we used for measurement of the filtration efficiency, which may further contribute to the different result. The enhanced filtration efficiency of salt-coated filters against influenza virus aerosols as compared to bare filters can be explained by the observed wetting of aerosols, favoring greater adhesion to salt-coated filters. Furthermore, the significant improvement in filtration efficiency resulted in complete protection of mice against lethal influenza aerosols, which demonstrates the high level of protection provided by salt-coated filters, outperforming currently used bare filters.Rapid loss of HA activity and viral infectivity on salt-coated filters can be explained by physical destruction of virus during recrystallization of coated salts. When the salt-coated filter is exposed to virus aerosols, salt crystals below the aerosol droplet dissolve to increase osmotic pressure to virus. Due to evaporation, the salt concentra- tion of the droplet significantly increases and reaches the solubility limit, leading to recrystallization of salt. As a consequence, virus particles are exposed to increasing osmotic pressure during the drying process and are phys- ically damaged by crystallization. As shown in Fig. 3e,f, the superior advantage of physically destroying the virus adsorbed to the salt-coated PP filters through natural salt crystallization process was further confirmed in vivo. According to previous reports, hyperosmotic stress (>541 mOsm) and crystallization induce membrane pertur- bation with irreversible deformation of the viral envelope and structural virus damage, respectively, resulting in infectivity loss of virus30,31. Therefore, our data support that the extensive level of infectivity loss associated with a salt recrystallization process caused by physical contact between virus aerosols and salt coating can be used in developing virus negation systems that are reusable without reprocessing.Similarly to CA/09 H1N1 aerosols, increased protection in vivo due to higher filtration efficiency of salt-coated filters compared to bare filters and deactivation of virus on salt-coated filters were observed following exposure to PR/34 H1N1 and VN/04 H5N1 (Fig. 4a,b). This suggests that salt-coated filters prevent virus penetration and destroy virus attached to the filter in a non-specific way. Furthermore, the performance of salt-coated fil- ters was not degraded by storage at 37 °C and 70% RH, demonstrating that salt recrystallization-based filters can ensure protection even under harsh environmental conditions. Notably, for demonstration of the concept of salt-recrystallization based virus deactivation system, NaCl salt was used, which has a critical RH of 75% at 30 °C35. However, salts with higher critical RH can be easily used, such as ammonium sulfate, potassium chloride and potassium sulfate, which have critical RH of 80%, 84% and 96.3% at 30 °C, respectively35. This suggests that salt-coated filters may be developed for specific environmental conditions.In conclusion, we demonstrated that the developed salt-recrystallization based filtration system provides high filtration efficiency and successfully deactivates multiple subtypes of adsorbed viruses. Moreover, we have shown that stability of the salt coating is not compromised by high temperature and humidity, which suggests safe use and long-term storage/reuse at such environmental conditions. Although our tests are based on exposure to dif- ferent types of influenza virus, the significance of these results for personal and public protective measures may be generally extended to enveloped respiratory viruses where infection and transmission can occur by aerosol. Our salt-coated filter unit can promise the development of long-term stable, versatile airborne pathogen negation system, without safety concerns. In fact, the destruction mechanism of viruses solely depends on the simple, yet robust naturally occurring salt recrystallization process, combining the destabilizing effects of salt crystal growth and concentration increase during drying of aerosols. This idea can be easily applied to a wide range of existing technologies to obtain low-cost, universal personal and public means of protection against airborne pathogens, such as masks and air filters in hospitals. Therefore, we believe that salt-recrystallization based virus deactivation system can contribute to global health by providing a more reliable means of preventing transmission and infec- tion of pandemic or epidemic diseases and bioterrorism.MethodsBare and salt-coated filter samples preparation. The commercial surgical masks had a three-ply structure. The middle layer is the filter media, whereas the inner and outer layers provide support and protect the filter against wear and tear. The metal nose clips and elastic ear loops were removed and circular samples (radius: 3 cm) were cut from the masks. The PP filters (middle layer) were isolated by removing the inner and outer protective layers (bare filters, Filterbare). The coating solution was prepared by dissolving sodium chloride (NaCl; Sigma Aldrich, St. Louis, MO) in filtered DI water (0.22 μm pore size; Corning, Tewksbury, MA) under stirring at 400 rpm and 90 °C, followed by the addition of Tween 20 (Fisher Scientific) to a final concentration of 29.03 w/v% of NaCl and 1 v/v% of Tween 20. To obtain the salt-coated filters, the mask bare PP filters were pre-wet to contain approximately 600 μL of coating solution by incubating overnight at room temperature. Any remain- ing dry areas were removed by applying gentle strokes with tweezers to the filters while immersed in the coating Scientific RepoRts | 7:39956 | DOI: 10.1038/srep39956 7

www.nature.com/scientificreports/solution. Subsequently, the filters were deposited in the desired volume of coating solution (0, 100, 300, 600, 900 and 1200 μL, of which corresponding membranes are abbreviated as Filterwet, Filterwet+100μL, Filterwet+300μL, Filterwet+600μL, Filterwet+900μL, and Filterwet+1200μL, respectively) on petri dishes (60 × 15 mm; Fisher Scientific) to control the amount of NaCl per unit area and dried in an oven (Isotemp Incubator, Fisher Scientific) at 37 °C for 1 day.Influenza virus preparation. Influenza viruses A/California/04/2009 (CA/09, H1N1), A/Puerto Rico/8/34 (PR/34, H1N1) and A/Vietnam/1203/2004 (VN/04, H5N1) were grown in 10-day old embryonated hen eggs, in which H5N1 virus was derived by reverse genetics from HPAI A/Vietnam/1203/200436. Influenza viruses were purified from allantoic fluid using discontinuous sucrose gradient (15%, 30% and 60%) layers following the pre- viously reported procedure37.Aerosols exposure to filters. For experiments involving aerosols exposure, an aerosol chamber (L × W × H = 145 × 145 × 150 mm; Emka Inc., Middletown, PA) was used (Fig. S11). It has a connection to the vacuum line and a circular aperture in the top wall (diameter: 22 mm) to exactly accommodate the cylindrical part (diam- eter: 20 mm, height; 20 mm) of the nebulizer unit that is below the aerosol generator (Aeroneb Lab Nebulizer System; Aerogen, Galway, Ireland). Bleach was used as trap between the chamber and the vacuum pump (Welch 2522C-10, 22 L/min; Niles, IL). The filters were placed on top of the chamber aperture and the nebulizer unit was inserted, ensuring the tight seal of the filters against the side of the aperture. 5 μL of virus stock were added to the nebulizer unit, aerosols (VMD 2.5–4 μm from manufacturer specifications) were generated for 30 sec and subsequently the desired vacuum level (3, 10 or 17 kPa) was applied, by manual control, three times in 1 sec cycles. Notably, in the case of bare filters, pressure was only applied for filtration efficiency tests.For all assays and analysis, suspensions of the filters were prepared as follows, unless otherwise indicated. To reconstitute virus adsorbed onto filters, virus-laden filters were immersed in 400 μL of sterilized DI water for about 5 min, and then removed after vortexing from the suspension. The virus suspension was centrifuged at 19,800 g and 4 °C for 10 min (Centrifuge 5810 R, Eppendorf, Hauppauge, NY), followed by resuspension of pellets in 70 μL of DI water to eliminate any interference from materials in supernatant during assays.Filtration efficiency tests. The filters were exposed to the virus aerosols at 3, 10 and 17 kPa and suspen- sions of the filters were obtained, as described above. The filtration efficiency was calculated as the ratio of the amount of virus (i.e., total proteins measured from the virus) reconstituted from the filter to that from the virus in the exposure aerosols. The concentration of virus in aerosols was determined by generating viral aerosols into a 15 mL centrifuge tube, containing 1 mL of DI water. After vortexing, virus concentrations (i.e., total protein concentration) were measured with bicinchoninic acid assay (BCA protein assay kit; Thermo Fischer scientific, Waltham, IL) with bovine serum albumin as a standard. In the case of virus reconstituted from salt-coated filters, virus-laden filter suspension was replaced with DI water prior to BCA assay.In vivo infection tests. Lethal infectivity of influenza viruses (CA/09 H1N1) was examined in 8 week old female inbred BALB/c mice (Nara Biotech; Seoul, Korea) by using the intranasal route. For bare and salt-coated filters, 12 mice per group were infected with individual penetration dosage of influenza virus through each filter. The penetration dosage of the virus through the filters (Filterbare, Filterwet, Filterwet+600μL, and Filterwet+1200μL) was calculated from the filtration efficiency at 10 kPa (near breathing pressure) using the relationship: penetration dos- age = virus dosage in lethal aerosol × penetration efficiency (%)/100, where penetration efficiency (%) = 100 − filtration efficiency (%). To examine the effects of the aerosolization process on the viral infectivity change, two mice groups were infected with a lethal dose of virus before and after aerosol formation, which served as negative control groups. Body weight changes and survival rate of mice were monitored daily for 15 days. Mice with body weight loss greater than 25% were euthanized. All animal protocols were approved by the Kyung Hee University (KHU) Institutional Animal Care and Use Committee (IACUC). All animal experiments and husbandry involved in this work were conducted under the approved protocols and guidelines of KHU IACUC. KHU IACUC oper- ates under National Veterinary Research and Quarantine Service (NVRQS), and animal welfare law and regula- tions of the WOAH-OIE (World organization for animal health).To test strain-dependent lethal infection behavior, mice (12 per group) were infected with the penetrated dos- age of viral aerosols (PR/34 H1N1 and VN/04 H5N1 viruses) through Filterwet+600μL at 10 kPa. Time-dependent body weight change was monitored in the same manner described above.Lung viral titer and lung inflammatory cytokine assays after infection. On day 4 after infection 6 mice of each group were sacrificed for the collection of lung samples. Lung virus titers were measured on six-well plates containing confluent MDCK cell monolayers. Inflammatory cytokines (IFN-γ) were determined using BD OptEIA mouse IFN-γ ELISA kit (BD Biosciences, San Jose, CA) following the manufacturer’s procedure.Test of viral infectivity change on filters. To investigate the effects of salt-coating on viral infectivity loss, lethal influenza aerosols were exposed to four different types of filters (Filterbare, Filterwet, Filterwet+600μL, and Filterwet+1200μL). Since Filterbare exhibited almost complete penetration upon pressure application, aerosols were exposed to the bare filter in the absence of pressure and samples were carefully handled to prevent mechanical agitation. To measure time-dependent stability change of virus, virus-laden filters were incubated at ambient conditions for 0, 5, 15, and 60 min after aerosol exposure, and suspended in DI water to reconstitute virus at each time point. In vitro stability of virus was characterized by measuring hemagglutinin activity (HA) and virus titers at the same concentration as lethal dose30. The conformational stability of antigenic proteins was characterized by measuring intrinsic fluorescence using 0.1 mg/mL of virus suspension38. To investigate morphological change of virus, lipid stability of viral wall was characterized by nile red fluorescence (Sigma Aldrich), a fluorescent lipid Scientific RepoRts | 7:39956 | DOI: 10.1038/srep39956 8

Of course, we would like more evidence but it is a choice of trying to prevent inhalation/transmission instead of breathing in an aerosol, which may be ranging from nanometre to millimetre diameter. Sneezed or exhaled aerosols tend to be in the micrometer and above range. (The virus is supposed to be 0.1 micrometer dia). These are not perfect but as we are short of proper N95 etc masks then a gain of small margin is worth considering.If these are likely or shown to be harmful then their use should be discontinued, but what is the worst case outcome of wearing these?A bit of salt in your mouth, nose or eyes?

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Dr narendra Kumar (April 21, 2020 11:13 PM)

can silk be a preferred fabric for face mask owing to itsi. nature of protein as its chemical constituent to adsorb COVID-19 virus through its S-proteinii. Its known antibacterial, anti fungal characteristics, andiii. porous nature of silk fiber in size<100nm to act as barrier to COVID-19 virus as additional protection?In my considered view face masks made out of readily available silk fabric either alone or with cotton or other fabric as the second layer should be ideal alternative material for home made masks for security personnel and common people.

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john coffer (April 24, 2020 2:45 PM)

IODINE! IODINE!! I treat my home made cloth mask with 1% Iodine cow teet dip solution. It is only $6 a gallon from the local ag supply store. My mask has a middle pocket that I can put an Iodine treated folded over white coffee filter into. I put a fresh one in before I go to town for what ever social interaction. The mask is a requirement now in NY to enter a store. Before masks, for the last 18 years, I have wiped the base of my nose with Tincture of Iodine to get Iodine vapors in my airways before entering any stores, church, etc. to a near perfect record at preventing colds and flu. Before that I used to get colds and flu two to three times a year. Twice getting serious pneumonia. Notice the Japanese who have had a very low Covid death toll compared to the rest of the world with out all the mandatory shut-down of their economy. They are really into their Iodine laced gargle/mouth wash called Heigi. Says right on the bottle that it helps prevent the flu. Iodine has the special distinction of all anti-viral substances of being harmless to healthy mammalian cells. Wake up world! Iodine is the trick!

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Chris Evans (April 24, 2020 3:01 PM)

I have not read in any review the importance of sanitizing ones hands before removing the mask from ones face. I do this to reduce transmitting anything from my hands to my face after exiting the grocery store and returning to my auto that now has hand sanitizer, two masks and wipes for groceries as I transfer groceries from shopping cart to empty bags in my auto.

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Ana Santana (April 25, 2020 10:09 AM)

Hi. Has anybody use polyester fabric in the diy mask? Is this effective?I know hospital will prefer cotton fabric mask but I have lots of polyester fabric at home and I want to help the nurses in hospitals making polyester fabric mask with filter pockets.

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Susan Munford (May 2, 2020 11:11 PM)

I initially heard that 100% cotton was the preferred fabric, but I just read a study from the university of chicago about fabrics used for masks. Using chiffon (a polyester) And/or flannel with a cotton creates an electrostatic barrier in addition to a physical. I picked up a rayon shower curtain today and made several masks, using care not to puncture fabric with pinholes. I used two layers of 400 count cotton fabric, one layer of flannel and the rayon front. I left a pocket for an H100 halyard surgical wrap liner/filter. To create a seal on the mask, use double sided fabric tape to adhere the mask to face. I’ve made hundreds of masks the last two weeks and this looks like the best protection yet.

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Pat Booth (May 6, 2020 7:06 AM)

HelloI’m making face masks with materials readily available at home, since we are in lockdown (in the U.K.) with no fabric or diy stores open. I have used very high thread count pillows as the inner layer with tight twill woven tea towels as the outer side and non woven J cloth type of reusable kitchen cloth sewn is as a filter layer. The Pellon interfacing suggested on one website is out of stock on the internet. Will the J-cloth type (these vary but I’ve found a dense one available at Tesco) work well? Is this a safe option? I haven’t come across the fabric bags (non woven) available in North America. I’ve found a suit bag that seems to be non woven and I’m also wondering if this would be a good bet, but the J-cloth type of material seems really dense. I’m just wondering because it’s designed to be absorbent, not repel fluid.I’d appreciate any advice.ThanksPat Booth

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Katharine Birkett (May 6, 2020 9:21 AM)

Thank you for discussing the importance of fit. For those who now need to wear a homemade mask, whatever the efficacy of the fabrics themselves, a good seal and fit can only help, and a poor fit will undo whatever "ingress" benefit the fabric could have provided on its own. I am not sure why masks are rigidly classified as falling only into "poor seal--egress droplet protection only" vs. "excellent seal and materials--medical-grade ingress protection." Could it be that this two-part classification reflects pre-pandemic realities? For myself, if I need to grocery shop in a pandemic, without taking away an N95 from a nurse, I would like to wear the best-fitting, best-sealing fabric mask I can, made out of the best-filtering fabric I can find. It won't be an N95, but if it's better than a surgical mask, it's better.

I've designed a mask that seals well--glasses don't fog and you can see the fabric moving in and out when you breathe--and have made it in a range of sizes so even men with the longest faces can have a mask that doesn't slide off of the nose. I made this mask for nurses and every nurse who has received one loves it. It's not a substitute for an N95, of course. But it uses the densest cotton fabric combination I could find that is still breathable. I think it is superior to a surgical-style mask out of two layers of loosely-woven cotton. Soon I will be working out a polypropylene version for a group I am part of.

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S. Spence (May 7, 2020 5:23 PM)

I have an interfacing material which is 80% polyester, 20% nylon, non-woven material. Can one and/or two layers of this be used safely between 2 layers of cotton material when making a mask, for non medical people, to help protect from the COVID-19?

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Elizabeth Moroney (May 13, 2020 3:03 PM)

My homemade masks consist of a quilted outer layer (cotton/poly batting/cotton), a middle vacuum cleaner bag layer and an inner T-shirt layer. I've also heard of others using poly interfacing in the middle layer. I made the metal nose piece from old Costco coffee bag closures. It's soft against my skin and fits very closely across my face. Much more comfortable and form fitting than the disposable "surgical" type.

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John Keynes (June 1, 2020 10:40 PM)

Wearing masks is necessay as well as avoiding unnecessary touch during this period. I bought some disposable masks and N95 from epidemiology.amotx.com in case I've to go to crowded places. Homemade mask is cool but I'm just afraid it may not work.

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Mitony (June 2, 2020 2:14 AM)

I am a mask producer. I'd share my idea, just for reference.to measure if the material works or not, we have to check if the filter efficiency is small enough to block the virus.e.g. N95/USA, FFP2/EU the filter efficiency >=95% is measured base on 0.3um filter.This way, cloth mask, it does not work at all... Its nothing help to prevent any kind of virus.we think till today, the best material is nano fabric, which filter efficiency reaches no less than 99%.

In year 2016, we joined a fair in China, at that time we already saw one nano fiber masks , it's very impressive, as it can be reused for almost one month, while filter efficiency keeps 99%. The company is called Suzhou Nice Sweater Clothes Co., Ltd. Their brand is called Nayi. I had one of their mask on their exhibition at that time. Anyone got interest, please try google this company, I can not find their name card anymore. Hope it helps.